PROJECT SUMMARY In bilingual environments, children must separately discover the regularities in each of their two languages. It has been suggested that this process may be easier when each language can be associated with a particular speaker (e.g., one parent speaks one language, while the other speaks the second). However, there have been no direct empirical tests of this proposal. The proposed experiments will test the principle behind this question, that reliable associations between speaker and language facilitate learning of linguistic structures, in both simulated and real bilingual environments. This research will therefore provide insights that are both crucial for the scientific understanding of bilingualism and of great interest to parents and educators seeking to improve language outcomes for bilingual children. Specific Aim 1 is to examine how infants make use of the presence of different speakers to inform their learning. Our studies will compare and contrast bilingual and monolingual infants? attention to the correspondence between speakers and the input they provide. Though it has been assumed that consistent associations make it easier for bilingual infants to separate the languages in their environment, there is no evidence demonstrating that infants track this association. Experiment 1 will provide the first empirical test of whether bilingual and/or monolingual infants keep track of the languages used by new speakers in their environment. In a preliminary study, we simulated two bilingual environments that varied in whether structure was systematically tied to a speaker. We found that monolingual infants showed no difference in their learning, suggesting that they did not benefit from the added consistency. Experiment 2 will test whether bilingual infants, whose home environment contains different cues, show enhanced learning when there are correlations between speaker and structure. Specific Aim 2 will apply these questions to real bilingual experience and ask whether consistent speaker cues facilitate bilingual language learning. Experiment 3 will use a lab-based manipulation to test whether bilingual toddlers are better able to learn new labels that are produced by speakers who consistently use a single language. In Experiment 4, we will explore these effects in real home environments. We will collect recordings from bilingual homes and ask whether toddlers who experience more consistent language use in their daily lives show enhanced vocabulary development. Together, these studies will provide the most rigorous test to date of whether bilingual children benefit from hearing individual speakers remain consistent in their language use and thus will inform future practices and interventions designed to promote dual language learning.